FIG. 28 is a general schematic diagram of a conventional image forming apparatus. The image forming apparatus shown in FIG. 28 is a laser printer or a digital copier. A laser beam emitted from a semiconductor laser unit 1009 is scanned by a rotating polygon mirror 1003, to form a beam spot on a photosensitive element 1001 which is a medium to be scanned through a scanning lens 1002, and exposes the photosensitive element 1001 to form an electrostatic latent image thereon. At this time, a photodetector 1004 detects a scanning beam in each line.
A phase synchronization circuit 1006 receives a clock from a clock generation circuit 1005, generates an image clock (pixel clock) of which phase is synchronized in each line, and supplies the image clock to an image processing unit 1007 and a laser drive circuit 1008. The semiconductor laser unit 1009 controls an emission time of a semiconductor laser according to the image data formed by the image processing unit 1007 and to an image clock of which phase is set for each line by the phase synchronization circuit 1006, and formation of the electrostatic latent image on the photosensitive element 1001 is thereby controlled.
In such a scanning optical system, variations of scanning speed cause fluctuations in an image, which leads to degradation of image quality. Particularly, in a color image, a dot offset of each color occurs in a main scanning direction, which leads to color shift, and the color shift causes degradation of color reproduction and degradation of resolution. Therefore, to obtain a high quality image, it is essential to correct the scanning speed variations.
The scanning speed variations (error) are roughly classified into those as follows. Each main factor of them is explained below.
(1) Error of Each Facet of Polygon Mirror (for Each Scanning Line) (Hereinafter, “Error of Each Facet” as Necessary)
A factor causing the scanning speed variations includes fluctuation in distance from a rotating axis of a polarized reflective surface of a polarizer such as a polygon mirror. In other words, the factor includes eccentricity of the polygon mirror, and facet precision of the facets of the polygon mirror. This type of error is an error with a periodicity of several lines, for example, the number of lines for the number of facets of a polygon mirror.
(2) Error Due to Variations in Average Scanning Speed
The average scanning speed indicates an average of scanning speed for each facet of the polygon mirror. A factor causing such scanning speed variations includes variations in rotational speed of the polygon mirror, and variations in a scanning optical system due to various environmental changes such as temperature, humidity, and vibration. The factor also includes variations in scanning speed due to chromatic aberration in a scanning optical system because an emission wavelength of a semiconductor laser, which is a light source, is changed caused by temperature variations or the like. These types of error are comparatively moderate variations.
(3) Error of Each Light Source
This type of error includes scanning speed variations which occur in a multi-beam optical system that has a plurality of light sources such as a semiconductor laser array and that concurrently scans a plurality of light beams by a common scanning optical system. The main factor of this type is variations in scanning speed due to chromatic aberration in a scanning optical system because there is a difference in emission wavelength of the light sources. Because the emission wavelength varies between the light sources, the error (2) may sometimes become different in the light sources. Furthermore, the difference may occur in scanning speed of a plurality of beams depending on assembly accuracy of the light sources.
(4) Error of Each Scanning Optical System
When an image forming apparatus includes a plurality of photosensitive elements and scanning optical systems, and supports multiple colors, the difference in scanning speed of the scanning optical systems largely affects image quality. The main factor of this type includes deformation due to low manufacturing accuracy and assembly accuracy of the parts for the scanning optical systems and also due to change in the parts with time. Furthermore, because the light sources are different from each other, the error (3) may occur. This error is such that the average scanning speeds are different from each other and further the errors (1) and (2) discretely occur. Although some of image forming apparatuses include those in which part of units in the scanning optical system is shared, light paths from respective light sources to each medium to be scanned (photosensitive element) are different from each other, and hence, this error is also included in this error (4).
There have been some problems as follows about conventional control technology for pixel clock frequency. More specifically, because frequency of a reference clock used for phase comparison is frequency for a line, this frequency is extremely low (one-thousandths to one-tenths of thousandths) with respect to the pixel clock oscillated. Therefore, it is not possible to ensure a sufficient open loop gain of PLL (Phase Locked Loop) and to obtain satisfactory control accuracy. Furthermore, this frequency is weak in disturbance and the clock frequency thereby fluctuates, and therefore, a clock with high precision cannot be generated.
As a method of correcting the errors in scanning speed, various technologies are disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-183600 and Japanese Patent Application Laid-Open No. 2004-262101.